Flexible Optical-Fiber Ribbon

ABSTRACT

An optical-fiber ribbon having excellent flexibility, strength, and robustness includes optical fibers having a sacrificial, outer release layer that facilitates separation of an optical fiber from the optical-fiber ribbon without damaging the optical fiber&#39;s glass core, glass cladding, primary coating, secondary coating, and ink layer, if present.

FIELD OF THE INVENTION

The present invention relates to optical-fiber ribbons and methods forproducing optical-fiber ribbons.

BACKGROUND

The amount of data transmitted over optical fiber cables is continuouslyincreasing worldwide. This is especially so in data centers because ofthe expansion of cloud computing, which requires that data be receivedand transmitted in limited physical space. As such, there is anincreasing demand for high-fiber-count and high-fiber-density opticalcables. Moreover, there is constant desire to reduce construction costsof access cable networks, making the reduction of optical-cable diameterand weight central to the use of existing facilities (e.g., undergroundducts,) to reduce installation costs. Another practical requirement isthe ability to mass-fusion splice optical fibers to shorten the timerequired for connecting cables. This means that there areseveral—possibly conflicting—demands, such as decreasing optical-cablediameters, increasing optical-fiber density, and improving optical-cableworkability. This is serious and difficult challenge for optical-cablemanufacturers.

To achieve easy workability, optical-fiber ribbons can preferentially bemass-fusion spliced to simultaneously make multiple optical fiberconnections. Conventional optical-fiber ribbons have the disadvantage ofrigidity, however, because of the application of a resin layer aroundthe optical-fiber assembly to keep the optical fibers in a parallelplane. This rigidity limits the possibility of increasing fiber densityin optical-fiber cables.

SUMMARY

Accordingly, it is an exemplary object of the present invention toprovide an optical-fiber ribbon having excellent flexibility, strength,and robustness to facilitate rolling or folding of the constituentoptical fibers in the ribbon-width direction. It is another exemplaryobject of the present invention to provide an optical-fiber ribbon thatcan be mass-fusion spliced to make multiple optical-fiber connections.It is yet another exemplary object of the present invention to providean optical-fiber ribbon from which individual optical fibers (e.g., atmost three optical fibers encapsulated with a matrix material) can beseparated without damaging adjacent optical fibers.

One or more of these objects may be achieved in a first inventive aspectby an exemplary method of making an optical-fiber ribbon, includingthese steps:

(i) arranging a plurality of optical fibers into a longitudinaloptical-fiber assembly (e.g., a planar an optical-fiber assembly),wherein the plurality of optical fibers are substantially parallel andrespectively adjacent to each other, and wherein each optical fiberincludes, from its center to its periphery, a glass core, a glasscladding, a primary coating, a secondary coating, and an outer layerformed of a first curable resin that is less than completely cured(e.g., partly cured or substantially fully cured);

(ii) applying a second curable resin to a surface of the optical-fiberassembly, wherein the second curable resin forms a plurality ofsuccessive elongated rectilinear beads configured to form bonds (e.g.,elongated bonds) between adjacent optical fibers in the optical-fiberassembly; and

(iii) passing the optical-fiber assembly with the surficial, elongatedrectilinear beads through a curing station to cure the second curableresin and to further cure the first curable resin.

One or more of these objects may be achieved in a second inventiveaspect by an exemplary optical-fiber ribbon that includes (i) aplurality of respectively adjacent optical fibers extending in alongitudinal direction and arranged in parallel to form an optical-fiberassembly (e.g., a planar an optical-fiber assembly) and (ii) a pluralityof successive elongated rectilinear beads of a second cured resin (i.e.,a cured second curable resin) arranged along the length of theoptical-fiber assembly. Typically, each bead is configured to form anelongated bond between two adjacent optical fibers in the optical-fiberassembly, and the cured second curable resin of each elongated bond iscoupled (e.g., chemically coupled) to a respective first cured resin(i.e., the cured first curable resin) of two adjacent optical fibers.

As noted with respect to the related method, each optical fiberincludes, from its center to its periphery, a glass core, a glasscladding, a primary coating, a secondary coating, and an outer layerformed of a cured first curable resin. (The partly cured orsubstantially fully cured first curable resin is cured during themanufacture of the optical-fiber ribbon, either before or concurrentlywith the curing of the second curable resin—the second curable resinbeing configured to bond or otherwise join adjacent optical fibers.) Assuch, corresponding embodiments of the optical-fiber ribbon hereindisclosed are applicable to the related method for making anoptical-fiber ribbon, and vice versa.

An exemplary optical-fiber ribbon according to the present inventionthus has multiple optical fibers arranged in parallel and connected withother optical fibers in the optical-fiber assembly via cured resinbeads. In some embodiments, a connection (e.g., a chemical coupling) iscreated between the first curable resin, which is the outermost coatinglayer of the optical fibers, and the second curable resin, which istypically applied to the optical-fiber assembly in elongated rectilinearbeads. For example, where the first curable resin is partly cured (e.g.,significantly less than fully cured), the concurrent curing of the firstcurable resin and the second curable resin provides increased bondingstrength between the second curable resin and the optical fibers' firstcurable resin. Conversely, where the first curable resin issubstantially fully cured, the subsequent curing of the second curableresin provides decreased bonding strength between the second curableresin and the optical fibers' first curable resin. The relative strengthof the coupling between the first curable resin and the second curableresin affects the robustness of the optical-fiber ribbon and the ease bywhich optical fibers can be separated from the optical-fiber ribbon.

In this regard, when an optical fiber is to be peeled or otherwiseremoved from the optical-fiber ribbon, no damage ought to occur to theprincipal structure of the optical fibers. Accordingly, it is preferredthat the separation (e.g., failure or rupture) occur (i) within theelongated beads formed by the cured, second curable resin, (ii) at theinterface between the cured, second curable resin and the cured, firstcurable resin, (iii) within the optical fiber's outer layer formed bythe cured, first curable resin, or (iv) at the interface between thecured, first curable resin and the optical fiber's next contiguouslayer, typically the secondary coating or, if present, an optional inklayer positioned upon the secondary coating. To maintain the integrityof the optical fiber, it would be undesirable if the point of failure orrupture during optical-fiber peel-off were to occur, for example, withinthe optional ink layer, the secondary coating, or at the secondarycoating's interface with the primary coating. This kind of peel-offfailure could be considered unacceptable damage to the optical fiber.

The foregoing illustrative summary, other objectives and/or advantagesof the present disclosure, and the manner in which the same areaccomplished are further explained within the following detaileddescription and its accompanying drawings.

Definitions

The following definitions are used in the present description and claimsto define the stated subject matter. Other terms not cited (below) areintended to have the generally accepted meaning in the field:

“Optical-fiber assembly” as used in the present description means: aloose arrangement of the plurality of parallel adjacent optical fiberswith no bonding between the fibers; the assembly has a width (W) andinterstices or grooves between adjacent optical fibers.

“Assembly width (W)” or “width (W)” as used in the present descriptionmeans: the assembly is formed of a number (N) of optical fibers, eachhaving a diameter (D) and a length (L), whereby the assembly has anominal width (i.e., W=D×N).

“Bond” as used in the present description means: a bead of a secondcured resin (i.e., a cured second curable resin) that bonds two adjacentoptical fibers over a bonding length (l). It should be noted that if two(or more) subsequent beads are applied after another within the samegroove connecting the same two adjacent optical fibers, these two (ormore) beads are considered to form together a bond with a bonding length(l) equal to the sum of the length of such subsequent beads.

“Bonding material” as used in the present description means: thematerial of which a bond is formed. This is the second cured resin—orwhen not yet cured—the second curable resin.

“Outer layer material” as used in the present description means: thematerial of which the outer layer is formed. This is the first curableresin that, depending on the stage of the process, is uncured, partlycured, or fully cured.

“Chemically coupled” as used in the present description means: thepresence of chemical covalent bonds formed by the simultaneous curingthe second curable resin and the partly cured first curable resin. Theseresins each comprise a plurality of chemically active groups that formcrosslinks (e.g., chemical bonds) during curing; because of thesimultaneous curing at the interface of the beads (i.e., comprising thesecond curable resin) and the outer layer (i.e., comprising the firstcurable resin), chemical covalent bonds form between the chemicallyactive groups present in the second curable resin (e.g., in the beads)and the partly cured first curable resin (e.g., in the outer layer).

“Stepwise pattern” as used in the present description means: a patternconstituted by a succession of beads over the plurality of opticalfibers, wherein the beads (of the succession of beads) are each timespaced apart in the width direction at a distance of one optical fiber.As such, the step of the stepwise pattern is one optical fiber. Wherethe optical-fiber assembly is formed by a number (N) of optical fibers,an individual stepwise pattern is constituted by a succession of (N−1)beads.

“Zig-zag like arrangement” as used in the present description means: anarrangement following the trace of a triangle wave. The zig-zag likearrangement in the present application is obtained by fitting a linethrough mid-points of the subsequent beads of subsequent stepwisepatterns.

“Saw-tooth like arrangement” as used in the present description means:an arrangement following the trace of a saw-tooth wave. The saw-toothlike arrangement in the present application is obtained by fitting aline through mid-points of the subsequent beads of subsequent stepwisepatterns.

“Pitch (P)” as used in the present description means: a length equal tothe recurrence of the stepwise pattern in the same width direction.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described hereinafter with reference to theaccompanying drawings in which embodiments of the present invention areshown and in which like reference numbers indicate the same or similarelements. The drawings are provided as examples, may be schematic, andmay not be drawn to scale. The present inventive aspects may be embodiedin many different forms and should not be construed as limited to theexamples depicted in the drawings.

FIG. 1 depicts in a perspective view a representative optical-fiberassembly.

FIG. 2a depicts in a perspective view an exemplary embodiment of aninventive optical-fiber ribbon having an intermittent, discontinuouszig-zag like arrangement. FIG. 2b depicts in a perspective view anexemplary embodiment of an inventive optical-fiber ribbon having anintermittent, discontinuous zig-zag like arrangement with a differentbonding length than the exemplary embodiment depicted in FIG. 2 a.

FIG. 3 depicts in a perspective view an exemplary embodiment of aninventive optical-fiber ribbon having a continuous zig-zag likearrangement.

FIG. 4a depicts in a perspective view an exemplary embodiment of aninventive optical-fiber ribbon having an intermittent, discontinuoussaw-tooth like arrangement. FIG. 4b depicts the exemplary embodiment ofFIG. 4a with a fitted saw-tooth line and pitch.

FIG. 5 depicts in a perspective view an exemplary embodiment of aninventive optical-fiber ribbon having a partly continuous saw-tooth likearrangement.

FIG. 6 depicts in a perspective view an exemplary embodiment of aninventive optical-fiber ribbon having a continuous saw-tooth likearrangement.

FIG. 7 depicts in a schematic representation an exemplary process forpreparing an optical-fiber ribbon having six optical fibers.

FIG. 8 depicts in a perspective, schematic representation anoptical-fiber ribbon having a zig-zag like arrangement.

FIG. 9 depicts in a perspective, schematic representation anoptical-fiber ribbon having a saw-tooth like arrangement.

FIG. 10 is a plan-view photograph of an optical-fiber ribbon accordingto an exemplary embodiment of the present invention.

FIG. 11 is a cross-sectional photograph of an optical cable unit beingprepared using 24 optical-fiber ribbons, each having 12 optical fibers.

FIG. 12a and FIG. 12b are photographs of undamaged optical fibers afterseparation from an optical-fiber ribbon according to an exemplaryembodiment of the present invention.

FIG. 13a and FIG. 13b are photographs of damaged optical fibers afterseparation from a comparative optical-fiber ribbon.

FIG. 14 is a photograph of an optical-fiber ribbon being subjected tomechanical tensile testing, namely a T-peel test.

DETAILED DESCRIPTION

Various aspects and features are herein described with reference to theaccompanying figures. Details are set forth to provide a thoroughunderstanding of the present disclosure. It will be apparent, however,to those having ordinary skill in the art that the disclosedoptical-fiber ribbons and methods for producing optical-fiber ribbonsmay be practiced or performed without some or all of these specificdetails. As another example, features disclosed as part of oneembodiment can be used in another embodiment to yield a furtherembodiment. Sometimes, well-known aspects have not been described indetail to avoid unnecessarily obscuring the present disclosure. Thisdetailed description is therefore not to be taken in a limiting sense,and it is intended that other embodiments are within the spirit andscope of the present disclosure.

In a first aspect the invention embraces a method of producing anoptical-fiber ribbon, such as the optical-fiber ribbons 100-600 depictedin FIGS. 1-6. Several exemplary embodiments of the method are discussed(below) with reference to the figures, including the process schematicdepicted in FIG. 7.

In a first exemplary step, a plurality of fibers 2 are fed (e.g., into adie 12) to provide a longitudinal optical-fiber assembly 3 in which theplurality of optical fibers are substantially in parallel andrespectively adjacent to each other. The exemplary process is depictedin FIG. 7 (processing from right to left) and the optical-fiber assembly3 is shown in FIG. 1. In an exemplary embodiment, shown in FIG. 1, theoptical fibers are arranged parallel in a plane. Each optical fibertypically has a substantially circular cross section. In some exemplaryembodiments, the outer layer of the plurality of optical fibers includesa partly cured first curable resin. In other exemplary embodiments, theouter layer of the plurality of optical fibers includes a substantiallyfully cured first curable resin. In alternative exemplary embodiments,the outer layer of the plurality of optical fibers includes a completelycured first curable resin.

In a second exemplary step, a second curable resin is applied from adispenser 14 (or similar dispensing device) to a surface, such as theupper surface of the optical-fiber assembly 3. See FIG. 7. Theapplication of the second curable resin leads to the second curableresin forming a pattern of a plurality of intermittently arranged beads4 along the upper surface of the optical-fiber assembly 3.

In a third exemplary step, the optical-fiber assembly with beads appliedthereon is passed through a curing station 16 for curing the secondcurable resin and, if the first curable resin is less than completelycured (e.g., partly cured or substantially fully cured) to further curethe first curable resin. See FIG. 7.

By way of non-limiting illustration, where the first curable resin ispartly cured, the concurrent curing of the first curable resin and thesecond curable resin provides increased bonding strength between thesecond curable resin and the optical fibers' first curable resin.Conversely, where the first curable resin is substantially fully curedsuch that little further curing is possible, the subsequent curing ofthe second curable resin provides decreased bonding strength between thesecond curable resin and the optical fibers' first curable resin. Asnoted, the relative strength of the coupling between the first curableresin and the second curable resin affects the robustness of theoptical-fiber ribbon and the ease by which optical fibers can beseparated from the optical-fiber ribbon.

Curing the partly cured first curable resin (or the substantially fullycured first curable resin) that forms the optical fiber's outer layer tothe second curable resin that forms the bead seems to affectoptical-fiber-ribbon robustness and ease of optical-fiber separationfrom the optical-fiber ribbon. In the resulting optical-fiber ribbon,the point of failure when removing an optical fiber preferably occurs(i) at the interface between the bead (i.e., formed by the secondcurable resin as cured) and the outer layer (i.e., formed by the firstcurable resin as cured), (ii) within the sacrificial outer layer itself(i.e., formed by the first curable resin as cured), or (iii) at theinterface between the outer layer (i.e., formed by the first curableresin as cured) and the secondary coating layer (or the optional inklayer, if present). The outer layer of the optical fiber (i.e., formedby the first curable resin as cured) can be considered a sacrificialrelease layer that facilitates the separation of an optical fiber fromthe optical-fiber ribbon without damaging the optical fiber's principalstructural parts, namely the glass core, the glass cladding, the primarycoating, the secondary coating, and the optional ink layer, if present.

In an exemplary method, each bead is arranged to form a bond between twoadjacent optical fibers over a bonding length (l). Typically, a bondconnects two adjacent optical fibers and a successive bond connects twoadjacent optical fibers, at least one of which differs from the opticalfibers bonded by the preceding bond. Typically, each bond is separatedin longitudinal direction from a successive bond by a bonding distance(d). In an exemplary embodiment, the bonding length is larger than thebonding distance (l>d).

FIG. 8 depicts in a perspective, schematic representation anoptical-fiber ribbon having six optical fibers and a zig-zag stepwisearrangement of the second curable resin. FIG. 9 depicts in aperspective, schematic representation an optical-fiber ribbon having sixoptical fibers and a saw-tooth stepwise arrangement of the secondcurable resin.

In an exemplary embodiment, before feeding (or otherwise arranging) theplurality of optical fibers to provide a longitudinal optical-fiberassembly, a first curable resin of the outer layer of each of theplurality of optical fibers is partly cured to a curing degree ofbetween 85 percent and 95 percent, such as between 88 percent and 92percent (e.g., about 90 percent cured) or between 91 percent and 94percent (e.g., about 92 or 93 percent cured), to provide optical fibershaving an outer layer of a partly cured first curable resin. In anexemplary embodiment, a degree of curing between 85 percent and 95percent means a degree of surface curing (i.e., the curing of theoutermost portion of the first curable resin of each optical fiber'souter layer).

In another exemplary embodiment, before feeding (or otherwise arranging)the plurality of optical fibers to provide a longitudinal optical-fiberassembly, a first curable resin of the outer layer of each of theplurality of optical fibers is substantially fully cured to a curingdegree of 95 percent or more (e.g., about 96, 97, 98, or 99 percentcured), to provide optical fibers having an outer layer of asubstantially fully cured first curable resin.

In an exemplary embodiment, the optical fibers are formed by providingoptical fibers each having, from its center to its periphery, a glasscore, a glass cladding, a primary coating, and a secondary coating, andapplying a first curable resin to form an outer layer. Typically, thefirst curable resin is then partly cured (e.g., about 85 percent to 90percent cured or so) or substantially cured (e.g., about 95 percentcured or so) to form the optical-fiber ribbon.

The percentage or degree of surface curing may be determined bymeasuring the peak area using Fourier Transform Infrared (FTIR) of thepeak of the chemically active group of the resin (e.g., the peak at 1410cm⁻¹ of an acrylate group for a UV-curable acrylate resin). This peakarea is then compared to a reference peak of a completely cured sample(e.g., a peak of a chemically active group, such as 810 cm⁻¹ or 1410cm⁻¹, is not present) and to a reference peak of a completely uncuredsample. The ratio of the relative peaks provides the degree of surfacecure.

In an exemplary embodiment, the outer layer of the first curable resinof each optical fiber is partly cured (e.g., 85 to 90 percent cured) inan environment including oxygen. If oxygen is present during curing, theouter surface of the outer layer does not fully cure. Typically, theamount of oxygen surrounding the outer layer during curing is between500 ppm and 3,500 ppm, such as between 1,000 ppm and 2,000 ppm.

In another exemplary embodiment, the outer layer of the first curableresin of each optical fiber is at least 90 percent cured (e.g.,substantially fully cured to more than 95 percent) in a controllednitrogen-purging environment. For example, the curing station may bepurged with industrial-grade nitrogen (e.g., 99.9 mole percent pure) toachieve a high-nitrogen environment (99 mole percent nitrogen). Absentsuch nitrogen purging, the outer layer of the first curable resin mayachieve a lower surface cure, which can sometimes result in excessivebonding (e.g., strong chemical bonding, such as covalent bonding) withthe second curable resin. This can hinder separation of one or moreoptical fibers from the optical-fiber ribbon without damaging theoptical fiber's primary coating, secondary coating, or ink layer.

In an exemplary embodiment, the second curable resin, which forms thebeads, is applied with a viscosity of between 100 cP and 1000 cP,typically between 100 cP and 400 cP. This allows a sufficient viscousmass to fill the grooves between adjacent optical fibers and will yield,after curing, an optical-fiber ribbon having a flush ribbon bead,thereby reducing possible stresses in the optical-fiber ribbon whenrolled or folded. If the viscosity is too low, the material is too thinand runny, and the adhesive will excessively flow between the opticalfibers and not form a consistent bond. The viscosity is measured using aBrookfield digital rotational viscometer Model DV-II with RV1 spindle at10 rpm. The viscosity may be measured at several different temperatures,such as at 23° C. and/or at 30° C. and/or at 40° C. and/or at 50° C.and/or at 60° C., to determine the optimal temperature for theapplication of the second curable resin material.

In an exemplary embodiment, the second curable resin is heated andapplied at a temperature of up to 60° C. (e.g., between about 23° C. and60° C.). If higher temperatures are used during the preparation of theoptical-fiber ribbons, thermal stress might occur in the optical fibers,leading to attenuation (e.g., at a wavelength of 1310 nanometers, 1550nanometers, and/or 1625 nanometers).

In an exemplary embodiment, the dispenser (or other dispensing device)oscillates in a direction transverse to the longitudinal direction ofthe optical-fiber assembly. The oscillating device can create a stepwisepattern on one side of the optical-fiber assembly. The tip of thedispenser may oscillate (e.g., vibrate) in a transverse direction at ahigh frequency, such as between about 100 Hz and 200 Hz. In an exemplaryembodiment, the dispenser oscillates in a direction transverse to thelongitudinal direction (i.e. in the width direction) of theoptical-fiber assembly, and the optical-fiber assembly is moved inlongitudinal direction, such as via reels. See FIG. 7.

In an exemplary embodiment, the dispenser may deliver the liquid resin(e.g., the second curable resin) in fine droplets to the movingoptical-fiber assembly. Because of surface tension, the liquid resinwill flow together to form elongated beads.

In an exemplary embodiment, the curing station emits UV radiation forcuring the beads of the second curable resin and for further curing thepartly cured first curable resin (or the substantially fully cured firstcurable resin) for the outer layer of the optical fibers.

In an exemplary embodiment, the first curable resin and/or the secondcurable resin are one or more curable ultraviolet (UV) resins. In anexemplary embodiment, the curable resins used are the same for the beadsand the outer layer. In an exemplary embodiment, the first curable resinis a UV curable ink having a pigment or dye for coloring. In anexemplary embodiment, a difference between the first curable resin andthe second curable resin is the amount of slip or release agent. Forexample, the first curable resin might include more than 0.5 weightpercent release agent or slip agent (e.g., between about 0.5 and 1weight percent or so), whereas the second curable resin might includeless than 0.5 weight percent release agent or slip agent, or none atall.

In a second aspect the invention embraces an optical-fiber ribbon100-600, such as depicted in FIGS. 1-6. Several exemplary embodiments ofthe optical-fiber ribbon are discussed (below) with reference to thefigures. In accordance with the present disclosure, the bonding strengthbetween the bead (e.g., formed by the second curable resin) and theoptical fibers can be controlled (e.g., via zig-zag like, saw-toothlike, or similar sinusoidal arrangements) to ensure optical-fiber-ribbonintegrity during damage-free handling and separation of individualfibers from the optical-fiber ribbon. This is achieved by the inclusionof a sacrificial release layer (e.g., an outer layer formed by the firstcurable resin) that facilitates the separation of an optical fiber fromthe optical-fiber ribbon without damaging the optical fiber's structuralcomponents, namely the glass core, the glass cladding, the primarycoating, the secondary coating, and the optional ink layer, if present.

FIG. 1 depicts in a perspective view a representative optical-fiberassembly 100. This optical-fiber assembly includes a plurality ofadjacent optical fibers 2 having a diameter D. The optical fibers arearranged substantially planar in parallel to form a longitudinaloptical-fiber assembly 3 having a width W and a length L. Thisoptical-fiber assembly 100 forms the basis for the optical-fiber ribbonaccording to the present invention.

In an exemplary embodiment, one or more bonds have a bonding length (l)and are spaced apart in a longitudinal direction by a distance (d). Forexample, the elongated bonds are substantially parallel to the opticalfibers in the optical-fiber ribbon. In this exemplary embodiment, thebonding length is larger than the distance (l>d). The effect is that themechanical properties in terms of robustness are increased, because alarger mechanical bond between the optical fibers is achieved.

In an exemplary embodiment, the bonding length is between about 2 and 20times the distance (2d≤l≤20d or l/d=2 to 20), wherein the values of 2and 20 are included. In another exemplary embodiment, the bonding lengthis between about 4 and 15 times the distance (4d≤l≤15d or l/d=4 to 15),wherein the values of 4 and 15 are included. The bead as applied has anelongated form and will flow into a groove between two adjacent opticalfibers. The elongated beads forming a bond may have a width between 75micrometers and 350 micrometers (e.g., between about 200 micrometers and275 micrometers, which is similar to the diameter of the opticalfibers).

In an exemplary embodiment, the bonding length (l) of a bead is between1.5 and 20 millimeters. The bonding length of the bead is effectivelydescribed by the ratio of bonding length to bonding distance (l/d) andby the ratio of pitch of the stepwise pattern to the width of theoptical-fiber assembly (P/W).

In an exemplary embodiment, each of the plurality of optical fibers hassubstantially the same diameter. In an exemplary embodiment, the opticalfiber has a diameter of between 240 micrometers and 260 micrometers,more typically about 250 micrometers. Alternatively, the optical fibersmay have a reduced diameter, such as between about 180 micrometers and230 micrometers. In an exemplary embodiment, the optical-fiber assemblyincludes between six and 36 optical fibers (including 6 and 36), such asbetween 12 and 24 optical fibers (including 12 and 24).

In an exemplary embodiment, the point of failure when removing anoptical fiber from the optical-fiber ribbon is in the bead (i.e., formedby the second curable resin as cured). In another exemplary embodiment,the point of failure when removing an optical fiber from theoptical-fiber ribbon is at the interface between the bead (i.e., formedby the second curable resin as cured) and the outer layer (i.e., formedby the first curable resin as cured). In yet another exemplaryembodiment, the point of failure when removing an optical fiber from theoptical-fiber ribbon is in the outer layer (i.e., formed by the firstcurable resin as cured). In yet another exemplary embodiment, the pointof failure when removing an optical fiber from the optical-fiber ribbonis at the interface between the outer layer (i.e., formed by the firstcurable resin as cured) and the secondary coating layer or an ink layer,whichever layer is contiguously surrounded by the outer layer (i.e.,formed by the first curable resin as cured).

In an exemplary embodiment, the optical fibers are optical fibershaving, in addition to the primary coating and secondary coating, an inklayer (e.g., an ink layer contiguously surrounding the secondarycoating) and an outer layer (e.g., formed by the first curable resin).In another exemplary embodiment, the outer layer itself may constitutethe ink layer. In such an exemplary embodiment, it is desirable that thepoint of failure occur either within the bead (i.e., formed by thesecond curable resin as cured) or at the interface between the bead(i.e., formed by the second curable resin as cured) and the outer layer(i.e., formed by the first curable resin as cured). Those havingordinary skill in the art will understand the different kinds of primarycoatings, secondary coatings, and ink layers, as well as the structuresand thicknesses thereof. This application hereby incorporates byreference commonly owned U.S. Pat. No. 8,265,442 for aMicrobend-Resistant Optical Fiber and U.S. Pat. No. 8,600,206 for aReduced-Diameter Optical Fiber.

In an exemplary embodiment, the beads are arranged on only one side ofthe optical-fiber assembly. For example, the beads are arranged only onthe upper surface of the optical-fiber assembly (i.e., when the opticalfibers are arranged in a ribbon-like manner rather than rolled up). Theoptical-fiber assembly can be viewed as a ribbon-like assembly definingan upper surface, a lower surface, and two side edges. The upper andlower surfaces are not completely flat, because they are formed of asubstantially parallel arrangement of optical fibers. As such, the upperand lower surfaces have parallel longitudinal grooves between adjacentoptical fibers. The beads are positioned within the grooves formedbetween adjacent optical fibers. Those having ordinary skill in the artwill understand the optical fibers may not be perfectly parallel butrather substantially parallel in practice.

In an exemplary embodiment, two successive beads of the plurality ofbeads are connected by a transition part of the cured second curableresin. In an exemplary embodiment the transition part is S-shaped (in aplan view). In an exemplary embodiment, each two successive beads of theplurality of beads are connected by a transition part of the curedsecond curable resin.

In an exemplary embodiment, a succession of alternating beads andtransition parts forms a thread, wherein at each longitudinal positionof the optical-fiber assembly there is at most one thread. In anexemplary embodiment, the thread has a mass (in grams) per 10,000 metersof between 60 and 120 dtex, such as between 75 and 110 dtex.

In an exemplary embodiment, each two successive beads of the pluralityof beads are free from each other in that no cured second curable resinconnects the two successive beads. In other words, there is no thread ofresin but merely individual beads.

In an exemplary embodiment, successive beads form a stepwise patternover the plurality of optical fibers, each step being one optical fiber.

In an exemplary embodiment, the first curable resin and/or the secondcurable resin are one or more curable ultraviolet (UV) resins. In anexemplary embodiment, the first cured resin and/or the second curedresin are acrylate resins. The first and second cured resins may be thesame or different. In an exemplary embodiment, the first curable resinis a UV curable ink including a pigment or dye for coloring. As noted,in an exemplary embodiment, a difference between the first curable resinand the second curable resin is the amount of slip or release agent. Forexample, the first curable resin might include more than 0.5 weightpercent release agent or slip agent (e.g., between about 0.5 and 2weight percent, such as about 1 weight percent), whereas the secondcurable resin might include less than 0.5 weight percent release agentor slip agent, or none at all.

In an exemplary embodiment, the cured second curable resin has anelongation at break of at least 150 percent, typically between 200percent and 300 percent, such as between 200 percent and 250 percent. Inan exemplary embodiment, the cured second curable resin has a modulus ofelasticity (or Young's modulus) of between 1 MPa and 50 MPa, such asbetween 10 MPa and 20 MPa. In this regard, elongation at break andmodulus of elasticity was measured using the following method: ASTMD638-14 (“Standard Test Method for Tensile Properties of Plastics”).

As noted, the outer layer (i.e., formed by the first curable resin ascured) may include release agent to facilitate release of an opticalfiber from the optical-fiber ribbon. Conventional ribbon matrixmaterials that are used to completely surround and encapsulate anoptical-fiber assembly include a certain amount of release agent tofacilitate breakout of individual fibers or splitting of a fiber ribbon.With respect to the present flexible optical-fiber ribbon according tothe present invention, a reduced amount of release agent is employed.Surprisingly, it has been observed that by reducing the amount ofrelease agent (e.g., the release agent in the second curable resin), thepoint of failure (e.g., the point of breakage) upon removing an opticalfiber shifts to the interface between the bead (i.e., formed by thesecond curable resin as cured) and the outer layer (i.e., formed by thefirst curable resin as cured) or to the outer layer itself.

In an exemplary embodiment, the thickness of the outer layer (i.e., thesacrificial release layer formed by the first curable resin as cured) isbetween 2 micrometers and 10 micrometers, such as between 3 micrometersand 5 micrometers or, more typically, between 5 micrometers and 10micrometers.

Ribbon robustness can be tested using a mechanical tester, such as atensile tester (e.g., Instron 5567). For example, in a T-peel test, asingle fiber (or a group of adjacent fibers) from an end of theoptical-fiber ribbon is clamped in a grip of the tensile tester (e.g.Instron 5567), while the remaining fibers from the same end of theoptical-fiber ribbon are clamped in the opposite grip of the tensiletester. See FIG. 14 (showing the performance of the T-peel test on anoptical-fiber ribbon). When both grips move transversely away from eachother, the maximum force (N) until separation of the single fiber (orgroup of fibers) from the remaining fibers determines the bondingstrength. In such a T-peel test, which is typically performed at STP(e.g., room temperature and atmospheric pressure), the force to break asingle bond (i.e., the required separation force) is measured. In anexemplary embodiment of the optical-fiber ribbon, the force required toseparate the optical-fiber ribbon in a T-peel test is between 0.01 N and0.1 N (e.g., between about 0.03 N and 0.1 N, such as between 0.05 N and0.07 N).

Moreover, ribbon robustness and durability can be further evaluated viawatersoak testing (herein referred to as “watersoak testing”). Forexample, after immersion in 60° C. water for at least 30 days (andtypically 60 or 90 days or more), bonding strength as measured by theaforementioned T-peel test should be at least 70 percent of the originalbonding strength (e.g., 75 percent or more), more typically at least 80percent of the original bonding strength (e.g., 85 percent or more).(During the T-peel test the optical-fiber ribbon is no longer immersedin the water bath.) Moreover, optical attenuation of each optical fiberin the optical-fiber ribbon should not increase by more than 0.5 dB/km,typically 0.1 dB/km (e.g., less than about 0.05 dB/km), as measured at awavelength of 1550 nanometers during the period of water immersionand/or after the period of water immersion.

To implement watersoak testing of the present optical-fiber ribbon, 600meters of loosely coiled optical-fiber ribbon is completely immersed ina bath of 60° C. water for at least 30 days (e.g., 60 days, 90 days, or125 days). At the end of the water-immersion period, the optical-fiberribbon is removed from the water reservoir and the bonding strength ofthe optical-fiber ribbon is measured by the T-peel test. After therequisite period of water immersion, the optical-fiber ribbons accordingto the present invention maintained bonding strength of more than 70percent of the original bonding strength (as measured by the T-peeltest).

As noted, optical attenuation (e.g., added loss) of each optical fiberin the optical-fiber ribbon can be periodically measured by opticaltime-domain reflectometer (OTDR) both during or after the waterimmersion period. The optical-fiber ribbons according to the presentinvention demonstrated optical attenuation for each constituent opticalfiber of less than 0.1 dB/km (as measured at a wavelength of 1550nanometers) both during and after the period of water immersion.

As explained previously, a connection (e.g., a chemical coupling) iscreated between the first curable resin, which is an outermost coatinglayer of the optical fibers, and the second curable resin, which istypically applied to the optical-fiber assembly in beads. Where thefirst curable resin is partly cured (i.e., less than fully cured), theconcurrent curing of the first curable resin and the second curableresin provides increased bonding strength between the second curableresin and the optical fibers' first curable resin. Conversely, where thefirst curable resin is substantially fully cured, the subsequent curingof the second curable resin provides decreased bonding strength betweenthe second curable resin and the optical fibers' first curable resin.The relative strength of the coupling between the first curable resinand the second curable resin affects the robustness of the optical-fiberribbon and the ease by which optical fibers can be separated from theoptical-fiber ribbon (e.g., the properties of optical-fiber-ribbonrobustness and optical-fiber separability are typically inverselyrelated).

By way of illustration, FIG. 12a and FIG. 12b are photographs of anundamaged optical fiber after separation from an optical-fiber ribbonaccording to the present disclosure. Here, the points of failure whenremoving the optical fiber from the optical-fiber ribbon appears to haveoccurred both within the outer layer (i.e., formed by the first curableresin as cured) and at the interface between the outer layer (i.e.,formed by the first curable resin as cured) and the optical fiber's inklayer. This shows the sacrificial outer layer (i.e., formed by the firstcurable resin as cured) is functioning as intended. Moreover, thesacrificial outer layer showed a high degree of strain-inducedelasticity by the tear force during the T-peel test, which allowed thematerial to withstand a large degree of elongation before break. Thisflexibility also facilitates robustness due to increased fracturetoughness of the outer layer (i.e., formed by the first curable resin ascured).

In contrast, FIG. 13a and FIG. 13b are photographs of damaged opticalfibers after separation from a comparative optical-fiber ribbon. Withoutbeing bound to any theory (and with reference to FIGS. 13a and 13b ),excessive bond strength between the first curable resin and the secondcurable resin (and the correspondingly high peeling force required toseparate the optical fibers) has resulted in not only separation of theink layer from a secondary coating but also separation of the primarycoating from the glass cladding, thereby exposing bare glass. Bycontrolling the bonding strength between the bead (e.g., formed by thesecond curable resin) and the outer layer (e.g., formed by the firstcurable resin), an acceptable balance may be achieved between therobustness of the optical-fiber ribbon, which is important during thecabling process, and the ease by which individual optical fibers can beseparated from the optical-fiber ribbon without damaging the opticalfiber's structural portions, namely the glass core, the glass cladding,the primary coating, the secondary coating, and the optional ink layer,if present.

In an exemplary embodiment, a first bead forming a first bond connects afirst pair of adjacent optical fibers while a successive bond formed bya successive bead connects a further pair of adjacent optical fibers.Here, at least one optical fiber of the further pair of adjacent opticalfibers differs from the optical fibers of the first pair of adjacentoptical fibers. In an exemplary embodiment, at each longitudinalposition of the optical-fiber assembly (e.g., along the resultingoptical-fiber ribbon), there is at most one bond.

In a first example of this embodiment, the beads will have a stepwisepattern. In an exemplary embodiment, at an end of the stepwise patternof beads, the bead that follows the last bead of the pattern starts asubsequent stepwise pattern in the same width direction. Typically, thesuccessive stepwise patterns are free from each other in that no curedsecond curable resin connects the two stepwise patterns. This successionof stepwise patterns may be repeated, typically over the length of theoptical fibers, to form a saw-tooth-like arrangement over the pluralityof fibers, (in a plan view). In an exemplary embodiment of thissaw-tooth like arrangement, the pitch (P) (i) is equal to the recurrenceof the stepwise pattern in the same width direction and (ii) is between10× and 100× the width (W) of the optical-fiber assembly, typicallybetween 15× and 80× the width (W) of the optical-fiber assembly.

FIGS. 4a and 4b depict an exemplary embodiment of an optical-fiberribbon 400 having a saw-tooth like arrangement in which none of thebeads 4 are connected and the plurality of beads is arranged as adiscontinuous line. The saw-tooth like arrangement has a constantrepetition that follows the trace of a saw tooth wave with a pitch (P)as illustrated in FIG. 4 b.

FIG. 5 discloses an exemplary embodiment of an optical-fiber ribbon 500having a saw-tooth like arrangement. The plurality of beads 4 isarranged as a partly continuous line of the second curable resin. Thecontinuous line starts with a first bead 4 being applied between thefirst and second optical fibers 2 at the distant edge. This continuousline continues over the top of the second optical fiber, with atransition part 9, to the groove between the second and third opticalfibers, and further on over the top of the third optical fiber, with atransition part 9, to the groove between the third and fourth opticalfibers, and so on and so on. The continuous line ends in the groovebetween the fifth and sixth (nearest) optical fibers. A new continuousline starts in the groove between the first and second optical fibers ata pitch P from the first continuous line (such as illustrated in FIG. 4b).

FIG. 6 discloses an exemplary embodiment of an optical-fiber ribbon 600having a saw-tooth like arrangement. The plurality of beads is arrangedas a continuous line of the second curable resin. The difference betweenthe optical-fiber ribbon 600 depicted in FIG. 6 and the optical-fiberribbon 500 depicted in FIG. 5 is a resin line 9′ between the bead 4between the fifth and sixth optical fibers 2 of the first saw-tooth likearrangement and the bead 4 between the first and the second opticalfibers 2 of the second saw-tooth like arrangement.

In another exemplary embodiment with a stepwise pattern, a firststepwise pattern is formed in a first width direction and, at the end ofthe stepwise pattern, a further stepwise pattern in the oppositedirection is formed. This succession of stepwise patterns may berepeated, typically over the length of the optical fibers, therebyforming a zig-zag like arrangement over the plurality of optical fibers(in a plan view). The plurality of beads is provided so the plurality ofrespectively adjacent optical fibers of the optical-fiber assembly, whenthe optical-fiber assembly is brought into a folded-out condition,extends in the same virtually flat plan. In an exemplary embodiment ofthis zig-zag like arrangement, the pitch (P) (i) is equal to therecurrence of the stepwise pattern in the same width direction and (ii)is between 14× and 140× the width (W) of the optical-fiber assembly,typically between 18× and 100× the width (W) of the optical-fiberassembly.

FIG. 2a discloses a first embodiment of an optical-fiber ribbon 100having a zig-zag like arrangement. In this exemplary arrangement, noneof the beads 4 are connected and the plurality of beads is arranged as adiscontinuous line. FIG. 2b discloses a second embodiment of anoptical-fiber ribbon 200 having a zig-zag like arrangement, thearrangement is shown by the black striped line connecting the middlepoints of the beads. The difference between the optical-fiber ribbon 200depicted in FIG. 2b and the optical-fiber ribbon 100 depicted in FIG. 2ais the shorter bonding length (l). In this arrangement, none of thebeads 4 are connected and the plurality of beads is arranged as adiscontinuous line.

FIG. 3 discloses a third embodiment of an optical-fiber ribbon 300having a zig-zag like arrangement. The plurality of beads 4 is arrangedas a continuous line of the second curable resin and having transitionparts (e.g., in a similar manner as depicted in FIG. 5 and FIG. 6). Thezig-zag like arrangement of the embodiments according to FIGS. 2a, 2b ,and 3 has a constant repeated arrangement that follows the trace of atriangle wave with a pitch (P) as illustrated in FIG. 2 b.

In an exemplary embodiment, the width (W) of the optical-fiber assemblyis between about 2 millimeters and 10 millimeters (e.g., between 2millimeters and 4 millimeters). The width (W) of the optical-fiberassembly is typically described as the number (N) of optical fibers eachhaving a diameter (D), whereby W=D×N. In practice, the optical fibersare substantially contiguous to one another, although some small gapsmay exist between adjacent optical fibers.

In an exemplary embodiment, at a certain longitudinal position over thewidth (W) of the optical-fiber assembly, there is one bond. In anexemplary embodiment, at each longitudinal position over the width (W)of the optical-fiber assembly, there is one bond. In other words, at onecertain longitudinal position there is only one bond between two opticalfibers, and there is no bond present between another set of two adjacentoptical fibers. This structure reduces the number of bonds andfacilitates increased flexibility.

FIG. 10 is a plan-view photograph of an optical-fiber ribbon accordingto an exemplary embodiment, namely an optical-fiber ribbon having azig-zag like arrangement with a continuous line of a cured resin.

The optical-fiber ribbon according to the present invention may be usedto form optical-fiber-cable units and optical-fiber cables. An exampleof such an optical-fiber-cable unit is shown in FIG. 11. This exemplaryoptical-fiber-cable unit has 24 ribbons of 12 optical fibers each. Thisoptical-fiber-cable unit packs 288 optical fibers into a highoptical-fiber density. Accordingly, in another inventive aspect, thepresent invention embraces an optical-fiber-cable unit including one ormore optical-fiber ribbons (also according to the present invention)surrounded by a polymeric sheath. The present invention further embracesan optical-fiber cable including one or more of the optical-fiberribbons or optical-fiber-cable units according to the present invention.

As explained (above), the flexible optical-fiber ribbon according to thepresent invention facilitates mass-fusion splicing to make multipleoptical-fiber connections while allowing optical fibers to be separated(e.g., peeled or otherwise removed) from the optical-fiber ribbonwithout damaging one or more optical fibers. According to exemplaryembodiments herein disclosed, this can be achieved by coupling (e.g.,chemical coupling) the beads to the outer layer of the optical fibers,thereby directing the point of failure during optical-fiber peel-offaway from the optical fiber.

Other solutions providing similar results are also part of the presentinvention. For example, another solution is decreasing the amount ofrelease agent that is present in the outer layer (e.g., the firstcurable resin), even when the outer layer is fully cured prior to theapplication of the beads (e.g., the second curable resin). This seems toshift the point of failure (i) to the interface between the bead and theouter layer, (ii) to the outer layer itself, or (iii) to the interfacebetween the outer layer and the secondary coating layer (or an optionalink layer). Yet another solution is to increase the modulus of thematerial of the beads (e.g., the modulus of the second curable resin ascured), thereby making the cured beads more brittle and thereby shiftingthe point of failure to the bead itself. In this way, the beads willbreak while keeping the integrity of the optical fiber's principalstructure.

To supplement the present disclosure, this application incorporatesentirely by reference the following commonly assigned patents, patentapplication publications, and patent applications: U.S. Pat. No.7,623,747 for a Single Mode Optical Fiber; U.S. Pat. No. 7,889,960 for aBend-Insensitive Single-Mode Optical Fiber; U.S. Pat. No. 8,145,025 fora Single-Mode Optical Fiber Having Reduced Bending Losses; U.S. Pat. No.8,265,442 for a Microbend-Resistant Optical Fiber; U.S. Pat. No.8,600,206 for a Reduced-Diameter Optical Fiber; U.S. Patent ApplicationPublication No. US2018/0031792 (published Feb. 1, 2018); InternationalApplication No. PCT/EP2017/067454 (filed Jul. 11, 2017); InternationalApplication No. PCT/EP2018/050898 (filed Jan. 15, 2018); InternationalApplication No. PCT/EP2018/050899 (filed Jan. 15, 2018).

Other variations of the disclosed embodiments can be understood andeffected by those of ordinary skill in the art in practicing the presentinvention by studying the drawings, the disclosure, and the appendedclaims. In the claims, the word “comprising” does not exclude otherelements or steps, and the indefinite article “a” or “an” does notexclude a plurality.

It is within the scope of this disclosure for one or more of the terms“substantially,” “about,” “approximately,” and/or the like, to qualifyeach adjective and adverbs of the foregoing disclosure, to provide abroad disclosure. As an example, it is believed those of ordinary skillin the art will readily understand that, in different implementations ofthe features of this disclosure, reasonably different engineeringtolerances, precision, and/or accuracy may be applicable and suitablefor obtaining the desired result. Accordingly, it is believed those ofordinary skill will readily understand usage herein of the terms such as“substantially,” “about,” “approximately,” and the like.

The use of the term “and/or” includes any and all combinations of one ormore of the associated listed items. The figures are schematicrepresentations and so are not necessarily drawn to scale. Unlessotherwise noted, specific terms have been used in a generic anddescriptive sense and not for purposes of limitation.

While various aspects, features, and embodiments have been disclosedherein, other aspects, features, and embodiments will be apparent tothose having ordinary skill in the art. The various disclosed aspects,features, and embodiments are for purposes of illustration and are notintended to be limiting. It is intended that the scope of the presentinvention includes at least the following claims and their equivalents:

1. A method of making an optical-fiber ribbon, comprising (i) arranginga plurality of optical fibers into a longitudinal optical-fiberassembly, wherein the plurality of optical fibers are parallel andrespectively adjacent to each other, and wherein each optical fiberincludes, from its center to its periphery, a glass core, a glasscladding, a primary coating, a secondary coating, and an outer layerformed of a first curable resin that is less than completely cured; (ii)applying a second curable resin to a surface of the optical-fiberassembly, wherein the second curable resin forms a plurality ofsuccessive elongated rectilinear beads configured to form bonds betweenadjacent optical fibers in the optical-fiber assembly; and (iii) passingthe optical-fiber assembly with the surficial, elongated rectilinearbeads through a curing station to cure the second curable resin and tofurther cure the first curable resin.
 2. The method according to claim1, comprising, before the step of arranging the plurality of opticalfibers into a longitudinal optical-fiber assembly, curing the firstcurable resin to between 90 and 95 percent cured as determined usingFourier Transform Infrared (FTIR) of the peak of the chemically activegroup of the first curable resin.
 3. The method according to claim 1,comprising, before the step of arranging the plurality of optical fibersinto a longitudinal optical-fiber assembly, curing the first curableresin to greater than 95 percent cured as determined using FourierTransform Infrared (FTIR) of the peak of the chemically active group ofthe first curable resin.
 4. The method according to claim 1, comprising,before the step of arranging the plurality of optical fibers into alongitudinal optical-fiber assembly, curing the first curable resin inan environment in which the oxygen concentration is between 500 ppm and3,500 ppm.
 5. The method according to claim 1, comprising, before thestep of arranging the plurality of optical fibers into a longitudinaloptical-fiber assembly, curing the first curable resin in a controllednitrogen-purging environment in which the nitrogen concentration is atleast 99 mole percent nitrogen.
 6. An optical-fiber ribbon, comprising:(i) a plurality of respectively adjacent optical fibers extending in alongitudinal direction and arranged in parallel to form an optical-fiberassembly, wherein each optical fiber includes, from its center to itsperiphery, a glass core, a glass cladding, a primary coating, asecondary coating, and an outer layer formed of a cured first curableresin; and (ii) a plurality of successive elongated rectilinear beads ofa cured second curable resin arranged along the length of theoptical-fiber assembly, wherein the beads are configured to formelongated bonds between adjacent optical fibers in the optical-fiberassembly, and wherein the cured second curable resin of each elongatedbond is coupled to the cured first curable resin of respective, adjacentoptical fibers.
 7. The optical-fiber ribbon according to claim 6,wherein the cured second curable resin of each elongated bond ischemically coupled to the cured first curable resin of respective,adjacent optical fibers.
 8. The optical-fiber ribbon according to claim6, wherein the first curable resin and the second curable resin arecurable ultraviolet (UV) resins.
 9. The optical-fiber ribbon accordingto claim 6, wherein, for each optical fiber, the outer layer formed of acured first curable resin comprises an ink layer.
 10. The optical-fiberribbon according to claim 6, wherein the cured second curable resinforming the beads has elongation to break of at least 150 percent asmeasured via ASTM D638-14
 11. The optical-fiber ribbon according toclaim 6, wherein the cured second curable resin forming the beads hasYoung's modulus of between 1 MPa and 50 MPa as measured via ASTMD638-14.
 12. The optical-fiber ribbon according to claim 6, wherein: afirst elongated rectilinear bead of a cured second curable resin forms afirst elongated bond connecting a first pair of adjacent optical fibers;and a second elongated rectilinear bead of a cured second curable resinforms a second elongated bond connecting a second pair of adjacentoptical fibers, wherein at least one optical fiber of the second pair ofadjacent optical fibers differs from the optical fibers of the firstpair of adjacent optical fibers.
 13. The optical-fiber ribbon accordingto claim 6, wherein, as measured by the T-peel test, the force requiredto separate one optical fiber from the optical-fiber ribbon is between0.01 N and 0.1 N.
 14. The optical-fiber ribbon according to claim 6,wherein, after 30 days of immersion in 60° C. water, the optical-fiberribbon maintains at least 70 percent of its original bonding strength asmeasured by the T-peel test.
 15. The optical-fiber ribbon according toclaim 6, wherein, after 30 days of immersion in 60° C. water, theoptical-fiber ribbon maintains at least 80 percent of its originalbonding strength as measured by the T-peel test.
 16. The optical-fiberribbon according to claim 6, wherein, after 90 days of immersion in 60°C. water, the optical-fiber ribbon maintains at least 70 percent of itsoriginal bonding strength as measured by the T-peel test.
 17. Theoptical-fiber ribbon according to claim 6, wherein, after 90 days ofimmersion in 60° C. water, the optical-fiber ribbon maintains at least80 percent of its original bonding strength as measured by the T-peeltest.
 18. The optical-fiber ribbon according to claim 6, wherein, after30 days of immersion in 60° C. water, optical attenuation for eachconstituent optical fiber in the optical-fiber ribbon increased by lessthan 0.1 dB/km as measured at a wavelength of 1550 nanometers.
 19. Theoptical-fiber ribbon according to claim 6, wherein, after 90 days ofimmersion in 60° C. water, optical attenuation for each constituentoptical fiber in the optical-fiber ribbon increased by less than 0.05dB/km as measured at a wavelength of 1550 nanometers.
 20. Anoptical-fiber-cable unit comprising one or more optical-fiber ribbonsaccording to claim 6.